This is the technique I use to build a roll cage. Not the only way to do it but it works for me. This gives you the basic structure, you can then add all the cross bars, down bars (kickers) head rest etc. as you desire. Only variation I usually make is I fabricate the rear kicker bars to lock in the position of the main roll hoop before measuring and fabricating the forward bars above the door and down the "A" pillar to the floor.